1. Field of the Invention
This invention relates to a thermal printer and particularly to a thermal serial printer for printing characters along a printing line by a thermal printhead mounted on a carriage which is movable in a reciprocating manner.
2. Description of the Prior Art
Thermal printers are well known in the art and they may be generally classified into two categories. In one category, which may be called a transfer type thermal printer, heat-sensitive ink ribbon is placed between a thermal printhead and a recording medium of plain paper and the ink on the ink ribbon is selectively transferred to the plain paper by the printhead. On the other hand, in the other category, which may be called a direct type thermal printer, use is made of heat-sensitive paper as a recording medium and characters are recorded directly on the heat-sensitive paper by the thermal printhead. In so-called hybrid thermal printers in which plain paper and heat-sensitive paper may be interchangeably used as a recording medium, if it is desired to use heat-sensitive paper, the heat-sensitive ink ribbon must be removed.
In prior art thermal printers, it was typical to mount a ribbon cassette containing therein heat-sensitive ink ribbon directly on a carriage movable along a platen in a reciprocating manner in order to set the heat-sensitive ink ribbon in position. With such a prior art structure, however, it is necessary to provide an ink ribbon advancing mechanism in the carriage itself so that the carriage provided with a printing mechanism tends to become complicated in structure and heavy in weight. For this reason, a driving mechanism needed to move the carriage along the platen requires a motor having a large capacity, which, in turn, requires large power consumption. In addition, the carriage necessarily becomes larger in size because the ribbon cassette is mounted thereon, which then causes the printer as a whole to be larger in size.
Another prior art thermal printer is shown in FIG. 1, in which a pair of ribbon spools 1 and 2 are provided on both sides of a printer frame 3 and heat-sensitive ink ribbon 4 is extending between the pair of spools 1 and 2 as passing through a printing section of a carriage 5. With such a structure, it is true that the carriage 5 may be made light in weight, but difficulty exists in setting the ribbon 4 in the printing position and removing the ribbon 4 from the printing position. That is, since the ribbon spools 1 and 2 are located far away one from each other and the ink ribbon 4, which is relatively thin, extends over a long distance, it is difficult to set or remove the ink ribbon 4. Particularly, it is extremely difficult to remove the ink ribbon 4 when a recording medium 6 is switched from plain paper to heat-sensitive paper because the ribbon spools 1 and 2 must be removed one by one and that portion of the ribbon extending between the spools 1 and 2, which includes used and non-used portions, must be wound into either one or both of the spools 1 and 2. However, since the ribbon 4 is relatively thin, that lead out portion tends to curl when the spools 1 and 2 are removed, and, thus, it is often the case that the lead out portion must be wound up into the take-up spool 2 when the ink ribbon 4 is to be set in position again. It is true that various ink ribbon cassettes are used in conventional impact printers; however, these ink ribbon cassettes cannot be applied to thermal printers.